1. Technical Field
The present invention relates to a sensing circuit and an optical detection circuit capable of detecting a luminance, a temperature, and the like, and a display device and an electronic apparatus using the same.
2. Related Art
In an optical sensor that uses an amorphous silicon thin film transistor as a photoelectric conversion element, a difference between a detected luminance and a true luminance gradually increases with time. This is because the phenomenon of photodegradation that the conductivity decreases with increasing integrated luminance occurs. In order to eliminate a deviation of the detected luminance caused by such photodegradation, for example, JP-A-6-350803 discloses using two equal photoelectric conversion elements.
Specifically, JP-A-6-350803 discloses a line image sensor in which photoelectric conversion elements 1 and 2, which have almost the same structure as a thin film transistor used in an active-matrix-driving type liquid crystal display or the like and have the same properties, are connected in series (paragraph 0045 and FIG. 2 in JP-A-6-350803). Irradiated light 100a that is emitted from a light source and is reflected from a surface of a document 104 is incident on the photoelectric conversion element 1. In addition, irradiated light 100b that is emitted from the same light source and is reflected from a reflection surface 106 is incident on the photoelectric conversion element 2 (FIG. 1B and paragraph 0041 in JP-A-6-350803). In the equivalent circuit shown in FIG. 2 in JP-A-6-350803, assuming that the electric potential of a power source is Vd, electric resistance of the photoelectric conversion element 1 is Ra, and electric resistance of the photoelectric conversion element 2 is Rb, the electric potential Vo of an image read signal that is an output of the line image sensor is expressed as the following Equation (paragraph 0048 in JP-A-6-350803).Vo=(Vd)/{1+(Rb/Ra)}
Here, the irradiated light 100a incident on the photoelectric conversion element 1 and the irradiated light 100b incident on the photoelectric conversion element 2 are emitted from the same light source. Accordingly, even if the radiation intensity of the light source changes, the electric potential Vo of the image read signal does not depend on the radiation intensity but depends only on the concentration of a surface of a document containing image information (paragraph 0049 in JP-A-6-350803). Moreover, in the case of a document generally used, a region of a white color having a high reflectance occupies about 95% of the whole area of the document. Accordingly, in order to make the irradiated light 100b, which has the same luminance as the irradiated light 100a incident on the photoelectric conversion element 1, incident on the photoelectric conversion element 2 when the white document is read, the reflection surface 106 having a high reflectance is provided such that the degree of photodegradation of the photoelectric conversion element 1 is made to be approximately equal to that of the photoelectric conversion element 2 (paragraphs 0053, 0054, and 0056 in JP-A-6-350803). For this reason, in above Equation, Rb/Ra (ratio of electric resistance between the photoelectric conversion elements 1 and 2) is constant if the concentration of the document surface is the same. Accordingly, even if electric conductivities of the photoelectric conversion elements 1 and 2 decrease due to photodegradation, the ratio of electric resistance between the photoelectric conversion elements 1 and 2 does not change and the potential Vo of the image read signal is not affected by photodegradation (paragraphs 0028 and 0051 in JP-A-6-350803).
In order to eliminate the deviation of the detected luminance caused by photodegradation by using the technique disclosed in JP-A-6-350803, it is necessary to make the degree of photodegradation of the photoelectric conversion element 1 approximately equal to that of the photoelectric conversion element 2. Regarding this point, JP-A-6-350803 discloses that the luminance of the irradiated light 100b incident on the photoelectric conversion element 2 is preferably set targeting a white color because about 95% of the whole area of a document generally used is a white color having a high reflectance. However, there are many cases in which a rate of a white color in a document to be read is less than 10%, for example, like a case of reading a color print image. In this case, the difference between the integrated luminance of the photoelectric conversion element 1 and the integrated luminance of the photoelectric conversion element 2 increases gradually and reaches the size not negligible eventually. That is, since the degree of photodegradation of the photoelectric conversion element 1 and the degree of photodegradation of photoelectric conversion element 2 cannot be maintained approximately equal, a precise sensor output can be obtained.
In addition, for example, in the case of an optical sensor that detects the luminance (ambient brightness) of environmental light, it is not necessary to read a document image unlike the line image sensor. In such an optical sensor, a deviation of a detected luminance caused by photodegradation cannot be corrected even if the same environmental light is incident on two photoelectric conversion elements having the same photoelectric conversion characteristics.